Generally, a jet-propelled engine means a heat engine which ejects high temperature gas burned in the engine and then uses its repulsive force to advance. This jet-propelled engine is commonly called ‘jet engine’, and may include a rocket engine having oxygen source necessary for combustion in a broad sense.
The jet engine is mostly used as a prime mover of an airplane, and classified into four types depending on its structure and function.
Firstly, there is a Turbojet, which compresses air inhaled from the atmosphere with an axial-flow type or centrifugal-flow type compressor, draws this compressed air into a burner so that fuels injected into the burner are burned, and then discharges high-temperature and high-pressure combustion gas toward a compression-driving turbine. That is, Turbojet is a prime mover which obtains propulsive force by jetting the gas, which has passed through the turbine, through jet nozzles.
This engine receives a great deal of air so as to control the temperature of combustion gas lower than a certain point in aspect of heat resistance of the turbine material, and thus there remains a lot of oxygen among the combustion gas. Therefore, in order to increase propulsive force, some turbojet engines are equipped with a long tail pipe and inject secondary fuel into the pipe for the purpose of after-burning.
Next, there is Turboprop which is a jet-propelled engine having a structure that a propeller is attached to the turbojet. Turboprop has similar configuration to the turbojet. But, energy of combustion gas in Turboprop is mostly converted into driving force of the propeller. Thus, Turboprop uses propulsive force of the propeller and jetting force together. Turboprop has performance between a propeller and a turbojet, and is suitable for an engine of a passenger airplane or a transport plane not requiring a high-speed flight.
There is also Bypass Jet which has an axial compressor instead of the propeller of turboprop. Bypass Jet ejects a part of compressed air through outer circumference of the combustion chamber together with combustion gas. This does not need a reduction gear, which is a factor of demerit of the turboprop. Bypass Jet also consumes very small fuel and is suitable for relatively fast transport planes.
As another type of jet-propelled engines, there is Ram Jet. If flying speed increases, atmosphere air is relatively flowed into the engine and then compressed due to its inertia. This is called “Ram effect”, and Ram Jet introduces the compressed air into the combustion chamber by using this Ram effect and then injects fuel thereto. Ram Jet ejects combustion gas through jet nozzles and then uses its repulsive force to advance.
In fact, Ram Jet is equipped with a diffuser to help inflow air to move slowly. Slowly moving air increases pressure in the diffuser, and thus the air is easily compressed to very high pressure. This engine has very simple structure and its performance is better as the plane moves faster. Thus, Ram Jet is suitable for a prime mover of a supersonic airliner moving at two or three times the speed of sound.
However, to obtain motive power, high-speed air is applied to Ram Jet from outside. Thus, there is designed a hybrid engine called “Turbo Ram Jet” which operates as a turbojet at a low speed and as a ramjet at a high speed.
As another type of jet-propelled engines, there is Pulse Jet. Pulse Jet has an automatic valve at the front of air inhalator. When a plane is flying, atmosphere air pushes the automatic valve to be opened and enters the diffuser. The air entering the diffuser loses its speed and makes the pressure in the combustion chamber increased. Then, the fuel is injected and burned, which makes the pressure in the combustion chamber more increased. This makes the automatic valve is closed. The combustion gas is ejected through jet nozzles to give propulsive force. If the combustion gas is ejected, the pressure in the combustion chamber is decreased and then air can be flowed in the combustion gas through the automatic valve.
This Pulse Jet has a feature that combustion is intermittently generated, compared with other engines in which combustion is continuously generated. Pulse Jet has simple structure, but it has disadvantages such as large fuel consumption and short lifecycle.
In such a jet engine, most needed is reliability. When evaluating its performance, there are considered three main factors: the propulsive force of engine should be great, compared with its weight; the propulsive force should be great, compared with its front surface area; and the fuel consumption should be low.
However, three conditions are rarely satisfied. Particularly, the turbojet generates serious noise and consumes too much fuel, so not frequently used in these days. The ramjet primarily used in high-speed airplanes has an advantage that it has simple configuration and gives great propulsive force, compared with its front surface area, but it consumes too much fuel.
In addition, the turbofan and the turboprop lower its fuel consumption rate by converting the energy generated in its basic turbojet engine into rotary energy of fan or propeller installed at the front. However, the turbofan and the turboprop disadvantageously give not to great propulsive force, compared its front surface area, since the fan or the propeller rotates among atmosphere air having low density. It is because there are generated many losses in the forwarding propulsive force when the fan or the propeller rotates among the low-density atmosphere air.
Furthermore, since giving the propulsive force toward the atmosphere air straightly flowing from the front, the turbofan and the turboprop demonstrate its function only at or below supersonic speed. At a speed above the supersonic speed, the fan or the propeller does not push the rushing atmosphere but disturb the flow of atmosphere air.
In addition, the fan and the propeller used in the turbofan and the turboprop have relatively big diameter, so the engine becomes bigger and heavier. In addition, its big size becomes an obstruction to its advancing due to friction with the atmosphere, and its weight also works as a burden of the engine itself.
Therefore, there still remains a need for a jet engine having small weight, great propulsive force to a front surface area and low fuel consumption rate.